Optical discs are widely used as inexpensive digital information recording media. For example, Blu-ray discs have a capacity of 25 gigabytes when including a single layer and 50 gigabytes when including two layers and can record a high-vision video content of good quality for about 2 to 4.5 hours. Accordingly, as the capacity of an optical disc increases, the value of a digital video content recorded in one optical disc increases and, hence, the protection of the copyright of a digital content to be recorded has become an absolutely critical technological problem.
However, nowadays, there is an overflow of fraudulent copies of digital contents from optical discs and illegal optical disc manufacturers such as pirate manufacturers. This leads to such a situation where the sound distribution of digital copyrighted works is hindered and no proper profit sharing is made to copyright holders.
Accordingly, for example, patent literature 1 discloses an information recording/reproducing method for irradiating laser light, whose recording state is on/off controlled by a second signal in a lower band than a first signal, two or more times to the same information track on a recording medium already recorded with the first signal while positions on the recording medium are synchronized, thereby recording the second signal in such a superimposition manner as to further change an optical change in a part irradiated with the laser light, and separating and reproducing the second signal at the time of reproduction.
According to patent literature 1, by recording the second signal on the information track recorded with the first signal in a superimposition manner, the management of the information track on the recording medium is realized without validating/invalidating the information on the information track recorded with the second signal or providing a special information track as a substitute for the information track to manage the position of the information track.
However, in the invention disclosed in patent literature 1, the second signal is recorded on the part pre-recorded with the first signal in a superimposition manner a plurality of times and superimposed recording is repeated until the second signal can be normally read. Accordingly, accuracy in reading the pre-recorded first signal deteriorates and a defect margin relievable by an error correction or the like deteriorates. Further, since laser irradiation to the same region is repeated two or more times to enable the normal readout of the second signal, a recording time is excessively long. It is also known in ordinary optical disc recording apparatuses that a tracking operation becomes unstable if laser of a recording power level is irradiated during a tracking control to concave and convex recording marks of a ROM disc such as a DVD-ROM. This is because a tracking method to a track groove in a recordable optical disc and a tracking method to concave and convex recording marks of a ROM disc normally differ.
A method for stably recording sub-information in the case where laser light of a recording power level is irradiated while a tracking control is executed to concave and convex recording marks of a general ROM disc formed with no tracking groove has not yet been disclosed up to now.
Patent Literature 1: Publication of Japanese Patent No. 2903422